1. Field of the Invention
The present invention relates to an ink jet recording apparatus using a liquid discharging head to discharge ink or other liquid. More precisely, the invention relates to a printing apparatus for making given prints on a recording medium such as paper, cloth, nonwoven cloth, or OHP sheet.
Particularly, the present invention is to provide a printing apparatus capable of executing a continuous printing for a long time or a printing apparatus effectively applicable to making prints continuously on a cloth as wide as more than one meter. More specifically, there are named a printer, copying machine, ink jet printing apparatus, facsimile apparatus, or other office equipment, and a textile printing apparatus or other large industrial equipment as those to which the present invention is applicable.
2. Related Background Art
As the conventional liquid discharging apparatuses, there are those which discharge ink droplets onto a recording medium to obtain images or those which discharge special liquid for the utilization thereof. An ink jet recording apparatus is to discharge ink droplets onto a recording medium to form images. Unlike electronic photography or others, it uses a lesser number of devices or equipment that may be required before the formation of images. As a result, there is a significant advantage that the ink jet recording apparatuses are capable of forming intended images more stably.
In general, however, a discharge unit for discharging liquid is structured extremely fine. Therefore, defective discharges tend to occur due to solidification of dyes or pigments mixed in a liquid or due to adhesion of foreign particles, hence creating a problem that a liquid discharging apparatus such as an ink jet recording apparatus may present defective recording in some cases. In order to avoid the occurrence of a problem of this kind, it is practiced at appropriate intervals that liquid is forcibly exhausted by means of suction, compression, or the like known as recovery means; the discharging area of the discharge unit is cleaned; or gas or fluid is ejected onto the discharging area of the discharge unit.
Meanwhile, for use of an ink jet recording apparatus, it is desired to execute recording in a higher quality and resolution in consideration of its nature as an excellent recording apparatus. Therefore, its image formation is performed by use of finer nozzles. The use of finer nozzles often results in the creation of the problems described above, which leads to the unstable recording, and degradation of the quality of recorded images. Included among the causes in this respect are, twisted recording brought by the unstable discharging direction of ink with minute displacement of impacting positions of ink droplets; non-discharges due to clogging of discharge ports (nozzles) by adhesion or mixture of dust particles or overly viscous ink; non-discharge due to heater wiring breakdown in the bubble jet method in which air bubbles are created in ink by use of electrothermal transducing elements (heaters) to discharge ink; and also, non-discharge due to the adhesion of ink droplets to the discharge port surface to cover the discharge ports.
Because of such non-discharge of the kind, the lines that are not recorded appear as white streaks in a recorded image along the scanning direction of a serial printer, hence degrading the quality of the recorded image significantly.
This kind of problem is more often encountered when the number of nozzles is increased to several hundreds or thousands in anticipation of the enhanced printing throughput. Proportionally to this attempt, the probability that abnormal nozzles occur is inevitably increased to make it more difficult to obtain perfect images.
Also, from the viewpoint of head fabrication, it has been required to provide all the nozzles in a normal condition without any defects. However, when the number of nozzles is increased as described above, the probability rate of defective nozzles is proportionally increased in the head's manufacture. The yield rate is thus decreased making it difficult to reduce its market price ultimately.
Also, in the conventional art, even if the perfect nozzles are used, the head becomes unusable when a malfunction takes place with just one nozzle of the many ones while in recording. Therefore, a printing apparatus provided with a multinozzle head having six to eight nozzles each often encounters abnormal nozzles, and produces defective prints whenever such condition is encountered. Furthermore, whenever an abnormal nozzle occurs, heads should be replaced, hence not only presenting the problem of costs, but also, idling the apparatus because of the inevitable suspension of its operation.
Also, not necessarily in recording by use of the ink jet recording method, when a recording element becomes unable to record due to its damages or the like brought to a recording apparatus which forms images on a recording medium by use of various recording elements, it should eventually continue recording in a state that part of recording dots is missing in an image being recorded, or suspend the recording temporarily for the replacement of recording heads, and restore the state so that the apparatus is able to record again.
As an invention to solve the problems described above, the applicant hereof has disclosed in Japanese Patent Laid-Open Application No. 6-79956 a method whereby to execute a complementary recording in the recording position of a nozzle where non-discharge has taken place. According to this disclosure, a serial scan is performed by use of a multinozzle head, while a given area is divided into portions for several scans, so that the method is comparable to that of recording by executing multiple scans. In this method, each recording is executed complementarily. In a recording position of the nozzle whose discharging has become incapable, a printing is performed by another scan to complement this condition; hence preventing the image quality from being degraded by any possible non-discharges.
There is also disclosed in Japanese Patent Laid-Open Application No. 6-79956 a recording structure to complement non-discharge nozzles with a separately arranged head with respect to image data unable to be recorded by the nozzles caused to malfunction due to non-discharge.
As described above, the invention proposed in the Japanese Patent Laid-Open Application No. 6-79956 is to attain printing by complementing non-discharge nozzles in order to reduce image defects (white streaks and twisting) caused by abnormal nozzles such as those presenting non-discharge. However, by use of the multiscan method disclosed in the application described above, the nozzles for the execution of the complementary recording are to print overlapping data. Therefore, the speed of such complementary printing should be switched to agree with the speed that allows the overlapped data to be printed. Here, the printing speed of the multiscan method should be reduced to almost a half because this method is to execute sub-scans per half a recordable width. In practice, assuming that malfunction such as non-discharge takes place with respect to some of the nozzles, not all the nozzles being in such unfavorable condition, there is essentially no alternative for this proposed method but to reduce the printing speed of the recording apparatus.
Also, in the application described above, a recording structure is disclosed to complement non-discharge nozzles by use of a head separately arranged for dedicated use of the image data related to the nozzles affected by non-discharges. In accordance with this structure, it is possible to print without reducing the printing speed. However, a head should be provided for the dedicated use of such complementary operation, which is needed only as far as a malfunction of nozzles, such as caused by non-discharges, takes place. Moreover, it is necessary to carry on additional maintenance of the discharging condition of the nozzles dedicated for the complementary use. Particularly when the number of nozzles is increased to effectuate a high-speed printing, the costs of the head become inevitably greater. Also, for a color printing apparatus, the heads dedicated for the complementary use should be arranged in accordance with the heads to be used for plural colors. Hence, not only the problem related to higher costs, but also the structural arrangement of the apparatus becomes still more complicated. There is also a significant problem that the apparatus itself should be made larger to provide this complementary arrangement.
Also, in accordance with Japanese Patent Laid-Open Application No. 5-301427, which is filed by the applicant hereof, the conditions immediately after printing are read by a sensor at the time of printing execution simultaneously in order to compute any difference in data to be printed; thus interpreting such differences caused by non-discharge if any obtained. The disclosed structure is arranged so that a complementary recording is executed by a proceeding scan subsequent to this computation or by a head separately arranged for complementary use to follow. Even with this structure, the problems described above have not been solved completely.
Also, in the specification of U.S. Pat. No. 5,124,720, a structure is disclosed for printing by use of only a group of heads, which does not include any non-discharge nozzles, when a non-discharging occurs. In accordance with this structure, printing is made by use of only a front half or rear half of the head without using the central part of it if non-discharge takes place in the central part of the head, for example. Therefore, if non-discharges occur in many positions, the usable portion of the head is reduced immediately. If this structure is adopted for a color printer, the usable portion of the printing heads is reduced extremely because non-discharges may be overlapped with the heads to be used for other colors. This presents a disadvantage that along with this reduction of usable part of heads, the printing speed drops down significantly.